The sleeping state, the Brilliant, is the letter u, the second element, from utkarṣa ('exaltation') or from ubhayatvā ('intermediateness'). He exalts, verily, indeed, the continuity of knowledge; and he becomes equal (samāna); no one ignorant of Brahma is born in the family of him who knows this.

In plain words: The sound U stands for the dreaming state, because it is in the middle and suggests rising up. Whoever understands this grows in knowledge, becomes balanced with all, and their family will not lack knowledge of Brahman, the ultimate reality.